1. Field of the Invention
The present invention relates to a method of semiconductor substrate fabrication, and more particularly, to a method of Group III-V compound substrate fabrication.
2. Description of the Related Art
The performance and lifetime of semiconductor devices such as laser diodes or light-emitting diodes are determined by their constituents and are greatly influenced by base substrates on which devices are integrated. Many approaches to manufacture high-quality semiconductor substrates have been made. With an increasing interest in Group III-V compound semiconductor substrates, the fabrication of a GaN substrate, which represents the Group III-V compound semiconductor substrate, has received considerable attention in recent years.
The key to fabricate a high-quality GaN substrate is to reduce the defect density in the GaN substrate. Also, the overall fabrication process should be simplified at low cost.
A general method of GaN substrate fabrication involves growing a GaN layer on a sapphire substrate and removing the sapphire substrate. As the thickness of the GaN layer grown on the sapphire substrate increases, the defect density in the GaN layer tends to decrease. For this reason, there is a need to grow the GaN layer on the sapphire substrate as thick as possible.
A hydride vapor phase epitaxy (HVPE) method, which has a relatively high growing rate, sublimation method, and metal organic chemical vapor deposition method are effective to grow a thick GaN layer on a sapphire substrate. However, the MOCVD method has too slow a film growing rate to grow a GaN layer having a thickness of tens of micrometers to a few hundred micrometers although it provides a high-quality GaN layer.
In a conventional method of fabricating a GaN substrate by HVPE, a silicon oxide layer is formed on a sapphire substrate as a mask, and a GaN layer is grown thereon by epitaxial lateral overgrowth (ELO) (referred to as a “first method” below). Alternatively, as shown in FIG. 1, a thick GaN layer 12 is directly grown on a sapphire substrate 10 without using a mask (referred to as a “second method” below).
In the first method, it is difficult to form a wide, thick GaN layer because stress is unevenly exerted on the GaN layer grown on the sapphire substrate. In the second method, because the sapphire substrate 10 and the GaN layer 12 have different thermal expansion coefficients, stresses act on the sapphire substrate 10 and the GaN substrate 12 in different directions, as shown in FIG. 2. In FIG. 2, reference numeral 10a denotes a tensile stress in the sapphire substrate 10, and reference numeral 12a denotes a compressive stress in the GaN layer 12, which acts in an opposite direction to the tensile stress. These stresses acting in opposite patterns cause cracks in the sapphire substrate 10 and the GaN layer 12. The GaN layer 12 is doped with silicon for conductivity. Accordingly, the GaN layer 12 is susceptible to crack due to the internal stress caused by silicon doping.